This invention relates to a power supply for a high-intensity discharge (HID) lamp that requires a modulated input signal for proper operation.
Certain types of electrical lamps operate more efficiently or effectively when driven by an input current signal that is both alternating and modulated in some manner. One example of such a lamp is a high-intensity gas discharge (HID) lamp, of the type used indoors or outdoors for general illumination of parking lots and buildings. Proper operation of such lamps involves driving the lamp with a signal that results in illumination that is both stable, and of uniform color. It is also desirable that such lamps be insensitive to orientation of the lamp, producing stable illumination of uniform color when the lamp is oriented vertically, horizontally, or in some other position.
As disclosed in U.S. Pat. No. 6,184,633 B1, the operational stability and uniformity of light generated by such HID lamps can be improved by driving the lamp with a swept frequency input signal that is either amplitude modulated, or sequentially alternated with a periodic change in the range of the swept frequency. In particular, an input signal is disclosed that includes a current frequency sweep from 45 to 55 kHz within a sweep time of about 10 milliseconds, in combination with either: a constant amplitude modulation frequency of about 24.5 Hz and a modulation index of 0.24; or a periodic sequential change in sweep frequency to about 12.3 kHz. In the ""633 B1 patent, the swept frequency and amplitude modulating signals are generated separately and combined to produce the signal actually fed to the HID lamp. The modulation frequency is predetermined and maintained as a constant.
U.S. patent application Ser. No. 09/620,357, commonly assigned to the original assignee of the present application, provides additional methods and apparatus for determining the optimum modulation frequency and/or sequential operating frequency, through sequential and periodic measurements of the HID lamp voltage at various amplitude modulating frequencies within a predetermined range of frequencies. Once the appropriate values are determined, the ""357 application utilizes the methods and apparatus disclosed in the ""633 B1 patent for producing the input waveform.
U.S. patent application Ser. No. 09/916,953, commonly assigned to the original assignee of the present application, provides a method for operating a power source adapted to supply an input signal having an alternating current and voltage to an electrical load in a manner providing a desired power spectrum to the electrical load. The method includes calculating a portion of the power spectrum being delivered to the electrical load by the input signal, and modifying the input signal in such a manner that the delivered power spectrum more closely matches the desired power spectrum. The step of calculating a portion of the delivered power spectrum may include monitoring and integrating the alternating current and voltage of the input signal. The method may further include storing a representation of the desired power spectrum in the power source.
Although the teachings of the above referenced patent and applications provide considerably improved performance with HID lamps, the cost and complexity of the power supplies and control methods that they teach may be prohibitive. What is needed, therefore, is an improved power supply and method adapted for driving an HID lamp, which provides the operational advantages of the prior art, but can be produced in a more straightforward manner.
My invention provides such an improved power supply and method, that produces stable illumination of uniform color and reduced sensitivity to lamp orientation, by driving a high-intensity discharge (HID) lamp with a carrier signal alternated between two very high frequency (VHF) frequencies at a modulation rate approximately equal the second longitudinal frequency of the lamp, and feeding the signal through a filter having an inverse amplitude relationship to carrier frequency, to produce an input signal to the lamp that is amplitude modulated in an inverse relationship to the change in carrier frequency at a modulation rate approximately equal to the second longitudinal resonance frequency of the lamp. The two VHF frequencies are selected from a range of frequencies over which acoustic resonances are substantially damped and lamp operation is stable.
Driving the lamp in this manner results in stable illumination of uniform color and reduced sensitivity to lamp orientation, without the additional circuitry required in prior power supplies for modulating amplitude of the signal driving the lamp. Elimination of such amplitude modulating circuitry, in a power supply according to my invention, reduces the complexity and cost of the power supply.
One form of my invention provides a method for operating a high-intensity discharge lamp by supplying the lamp with an input waveform having a carrier frequency that is alternated at a modulation frequency between a first carrier frequency and a second carrier frequency higher than the first carrier frequency, and filtered by a filter circuit having an inverse amplitude relationship to carrier frequency, to thereby provide an input waveform that is amplitude modulated at the modulation frequency between a first amplitude at the first carrier frequency and a second amplitude lower than the first amplitude at the second carrier frequency. The method may include exciting a longitudinal acoustic mode in the lamp at a longitudinal resonance frequency by modulating the carrier frequency at a modulation frequency that is approximately equal to the longitudinal resonance frequency of the lamp. Where the high-intensity discharge lamp has a second longitudinal resonance frequency, the modulation frequency may be approximately equal to the second longitudinal resonance frequency of the lamp.
The method may include determining the longitudinal resonance frequencies of the lamp. The method may also include determining values for the first and second carrier frequencies, or a range of values bounded by the first and second carrier frequencies, at which illumination from the lamp is stable. The first and second carrier frequencies may be VHF frequencies.
Another form of my invention provides an apparatus for operating a high-intensity discharge lamp. The apparatus may include means for supplying the lamp with an input waveform having a carrier frequency alternated at a modulation frequency between a first carrier frequency and a second carrier frequency higher than the first carrier frequency, and a filter having an inverse amplitude relationship to carrier frequency operatively attached for filtering the input waveform to provide a waveform for driving the lamp that is amplitude modulated at the modulation frequency between a first amplitude at the first carrier frequency and a second amplitude lower than the first amplitude at the second carrier frequency.
Where the lamp has a longitudinal acoustic mode that is excitable at a longitudinal resonance frequency, the modulation frequency may be approximately equal to the longitudinal resonance frequency of the lamp. Where the high-intensity discharge lamp has a second longitudinal resonance frequency, the modulation frequency may be approximately equal to the second longitudinal resonance frequency of the lamp. The first and second carrier frequencies may be VHF frequencies at which the lamp illumination is stable.
The means for supplying the lamp with an input waveform may include a bridge circuit, and a controller for controlling the bridge circuit to produce the input signal. The filter may be a simple LC circuit, or a higher order circuit providing increased attenuation of the input signal as a function of frequency.
The foregoing and other features and advantages of my invention will become further apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of my invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.